The invention relates to a method and an apparatus for producing and stacking sheets cut from a web of material, in particular a cardboard web.
In systems for producing cardboard sheets of a desired format, a continuous web of material is first produced and then cut apart into sheets of the desired format by means of cutting devices. This requires at least one longitudinal cutter device for cutting individual web sections out of the continuously fed web and at least one crosswise cutter device, which from the web sections cuts the final sheets of the desired format.
These sheets are then delivered in the form of stacks by means of a stacking apparatus. One such stacking apparatus for stacking cardboard sheets as a rule includes one feeder apparatus per web section, on which feeder apparatus the individual sheets, cut from the applicable web section and after being cut crosswise, are delivered in the form of an offset or staggered stream of sheets overlapping one another.
Each stream of sheets is thus fed to a stacking device, also called stacker. In such a stacker, the sheets are combined into stacks of predetermined height, and once the desired stack height or the desired number of sheets per stack is reached, the stack is conveyed out of the stacker, so that the next stack can be created. A stacking apparatus of this kind is described for instance in European Patent Disclosure EP 0 211 996 A1.
Since it takes a certain length of time to remove a finished stack from the stacker, a sufficient gap must be created in the stream of sheets fed to the stacker, because otherwise the result would be a defective stacking process. However, the requirement to form a gap in the stream of sheets limits the production speed of the overall system.
Stacking apparatuses with two stackers have therefore been created, between which it is possible to switch back and forth by means of a shunt disposed in the feeder apparatus.
In this way the advantage is attained that the gap to be created in the stream of sheets need correspond only to a period of time that the shunt requires to switch over from one stacker to the other.
If not only a single web section of predetermined width but instead at least two web sections, which it is understood may be of different widths, are cut from the continuously produced cardboard web, then typically a stacking apparatus is used that has one feeder apparatus for the sheets cut from each web section. Thus at least one stacker for each feeder apparatus must be provided.
For optimal utilization of the continuously produced cardboard web, it is necessary that the individual jobs for each web section be associated with one another as much as possible such that the least possible amount of blanking waste occurs. A job is defined by a predetermined number of sheets of the same format. One job therefore corresponds to a certain total length of one web section.
Until now, when at least two partial jobs were being executed simultaneously from an originally single web of cardboard, the procedure was such that from a plurality of jobs to be executed, the jobs to be executed in parallel were selected in such a way that both upon the transition from one job for one web section to the applicable other job and during the execution of two jobs, the least possible blanking waste occurs.
If the jobs to be executed with the individual web section end at very different times or at correspondingly different locations with respect to the web length, and if the job that ends earlier is followed by a job with a shorter sheet width, then typically this is a point of major blanking waste.
Moreover, this known method has the disadvantage that in the event that very large jobs are to be executed with both web sections, there is no possibility of slipping in a higher-priority job involving at least one web section in between.